1. Field of the Invention
The present invention relates to a technique to correct a color output from a device.
2. Description of the Related Art
Image output devices are roughly classified into intensity modulation (density modulation) type output devices and dot area modulation type output devices. There are various intensity modulation type output devices. Typical examples are a CRT, liquid crystal display, and sublimation printer compatible with multi-bit output. On the other hand, there are various dot area modulation type output devices. Typical examples are an inkjet printer and an electrophotographic printer.
The intensity modulation type output device expresses grayscale by the luminance level of each pixel. In the output of the intensity modulation type output device, each pixel does not overlap an adjacent pixel. The intensity modulation type output device is designed not to easily change the size of a pixel depending on the luminance level. As is known, if adjacent pixels overlap, or the size of a pixel changes in accordance with the luminance, the luminance characteristic becomes nonlinear. In the intensity modulation type output device, if the average pixel value of image data is the same, the output luminance characteristic hardly changes even when the luminance pattern (spatial frequency characteristic) has changed.
On the other hand, the dot area modulation type output device expresses grayscale by the area of each pixel. Especially in an inkjet printer or electrophotographic printer, an ink dot or toner dot corresponding to a pixel is designed to be relatively large with respect to the output resolution. Dots that are designed relatively large readily overlap. As is known, if the dots overlap, the luminance characteristic generally becomes nonlinear. Hence, in principle, in the inkjet printer or electrophotographic printer, even if the average pixel value of image data is the same, the output luminance characteristic readily changes when the dot pattern (spatial frequency characteristic) has changed.
Recently, a correction technique to match the color of a dot area modulation type output device with that of an intensity modulation type output device (color matching) has been proposed. A correction technique of matching the color of an intensity modulation type output device with that of a dot area modulation type output device and a correction technique of matching the color of a dot area modulation type output device with an ideal color have also been proposed. For example, Japanese Patent Laid-Open No. 7-333822 discloses a technique of performing color correction of the above-described dot area modulation type output device such as a printer. According to the invention described in Japanese Patent Laid-Open No. 7-333822, when printing using a plurality of kinds of dots with different print characteristics, the tone characteristic of an image signal is corrected in accordance with the dot gain characteristic.
According to the invention described in Japanese Patent Laid-Open No. 7-333822, a high color correction accuracy can be obtained for an image region whose spatial frequency is almost zero. However, when an image contains high-frequency components and has a changeable pixel value distribution (for example, contrast, average value, and histogram), the color correction accuracy degrades.
For example, FIG. 1A illustrates images output by a liquid crystal display that is an intensity modulation type output device. In FIG. 1A, reference numerals 1001 and 1002 denote input images each having different spatial frequencies in the X direction. In the example of FIG. 1A, the spatial frequency of the input image 1001 is 12.5 dpi, and the spatial frequency of the input image 1002 is 100 dpi. FIG. 1B shows average luminances 1101 and 1102 at this time. As is apparent from FIG. 1B, even when the spatial frequencies are different, the average luminance is not different. In FIG. 1A, reference numerals 1003 and 1004 denote input images each having different spatial frequencies in the Y direction. The spatial frequency of the input image 1003 is 12.5 dpi, like the input image 1001. The spatial frequency of the input image 1004 is 100 dpi, like the input image 1002. As is apparent from FIG. 1B, even when the spatial frequencies are different, the average luminance is not different in this case as well.
FIG. 2A illustrates images output by an inkjet printer that is a dot area modulation type output device. The images shown in FIG. 2A use the error diffusion method as halftone processing. As the error diffusion method, conventional processing using an appropriate γ table is performed. In FIG. 2A, reference numerals 1201 and 1202 denote input images each having different spatial frequencies in the main scanning direction (X direction), as in FIG. 1A. In the example of FIG. 2A, the spatial frequency of the input image 1201 is 18.75 dpi. The spatial frequency of the input image 1202 is 600 dpi. FIG. 2B shows average luminances 1301 and 1302 in this case. As is apparent from FIG. 2B, when the spatial frequencies of input images are different, the average luminances are also different. In addition, if the input image has a high frequency, the output image becomes dark.
In FIG. 2A, reference numerals 1203 and 1204 denote input images each having different spatial frequencies in the sub-scanning direction (Y direction). FIG. 2B shows average luminances 1303 and 1304 in this case. As is apparent from FIG. 2B, when the spatial frequencies of input images are different, the average luminances are also different, and if the input image has a high frequency, the output image becomes dark even in the sub-scanning direction (Y direction). Furthermore, the luminance characteristic is different from that in the main scanning direction (X direction), as can be seen.
A possible cause of the above problems is that the γ table used in the above-described technique is generated based on data obtained by measuring a plurality of color patches by a colorimeter or the like. In other words, a possible cause is that the correction is suitable for a region whose spatial frequency is almost zero. As a result, when the spatial frequency, frequency direction, and pixel value distribution (for example, contrast, average value, histogram, minimum value, and maximum value) of an input image change, the color correction accuracy degrades.